Summary of Work: Our previous work has shown that nucleotide excision repair is defective in the Li-Fraumeni syndrome, a heritable cancer prone syndrome associated with increased susceptibility to breast cancer. Other evidence suggests that defective DNA repair may also be a susceptibility factor in the development of sporadically occurring breast neoplasms as well. Nucleotide excision repair has been studied in normal human mammary epithelial tissue and in hormone dependent and independent breast tumor cell lines to examine which DNA repair mechanisms play be implicated in breast tumorigenesis. UV-induced DNA damage has been used as the prototypic DNA damaging agent to assess the efficiency of overall genome repair as well as transcription coupled repair. In addition, hypersensitivity to UV light has been evaluated. Cell survival assays suggest that the hormone-independent cell line is exquisitely hypersensitive to ultraviolet light. This UV-hypersensitivity is associated with an oligonucleosomal DNA fragmentation pattern closely resembling the pattern shown as a result of programmed cell death induced in these cells by chemotherapeutic agents. UV hypersensitivity resulting in apoptosis has not been previously reported in breast cancer cells. In contrast, the normal human mammary epithelial (HMEC) cell line and the hormone responsive tumor cells have UV-sensitivity levels comparable to normal human cell lines. Studies of bulk DNA repair reveal a defect in the processing and repair of UV-induced cyclobutane pyrimidine dimers for both breast cancer lines. Furthermore, results of gene-specific repair experiments performed on both tumor cell lines, the normal HMEC, and normal human fibroblasts suggest that transcription-coupled repair in the two tumor cell lines is defective. By expanding our repair studies to benign and malignant materials from older women, we hope to identify the particular DNA repair pathways involved in tumorigenesis in the senescent mammary gland. This information may allow identification of women at risk as well as to design better treatment plans for older breast cancer patients whose ability to tolerate standard chemotherapeutic regimens is frequently impaired.